Mouse Otx2 functions in the formation and patterning of rostral head

The anterior part of the vertebrate head expresses a group of homeo box genes in segmentally restricted patterns during embryogenesis. Among these, Otx2 expression covers the entire fore- and midbrains and takes place earliest. To examine its role in development of the rostral head, a mutation was introduced into this locus. The homozygous mutants did not develop structures anterior to rhombomere 3, indicating an essential role of Otx2 in the formation of the rostral head. In contrast, heterozygous mutants displayed craniofacial malformations designated as otocephaly; affected structures appeared to correspond to the most posterior and most anterior domains of Otx expression where Otx1 is not expressed. The homo- and heterozygous mutant phenotypes suggest Otx2 functions as a gap-like gene in the rostral head where Hox code is not present. The evolutionary significance of Otx2 mutant phenotypes was discussed for the innovation of the neurocranium and the jaw.

Otx2 homeobox gene controls retinal photoreceptor cell fate and pineal gland development

Understanding the molecular mechanisms by which distinct cell fate is determined during organogenesis is a central issue in development and disease. Here, using conditional gene ablation in mice, we show that the transcription factor Otx2 is essential for retinal photoreceptor cell fate determination and development of the pineal gland. Otx2-deficiency converted differentiating photoreceptor cells to amacrine-like neurons and led to a total lack of pinealocytes in the pineal gland. We also found that Otx2 transactivates the cone-rod homeobox gene Crx, which is required for terminal differentiation and maintenance of photoreceptor cells. Furthermore, retroviral gene transfer of Otx2 steers retinal progenitor cells toward becoming photoreceptors. Thus, Otx2 is a key regulatory gene for the cell fate determination of retinal photoreceptor cells. Our results reveal the key molecular steps required for photoreceptor cell-fate determination and pinealocyte development.

Defects of urogenital development in mice lacking Emx2

The homeobox gene Emx2 is a mouse homologue of a Drosophila head gap gene empty spiracles (ems) and is essential for the development of dorsal telencephalon (Yoshida, M., Suda, Y., Matsuo, I., Miyamoto, N., Takeda, N., Kuratani, S. and Aizawa, S. (1997) Development 124, 101–111). At the same time, Emx2 is expressed in the epithelial components of the developing urogenital system and, in Emx2 mutant mice, the kidneys, ureters, gonads and genital tracts were completely missing. Pax-2 and c-ret expressions in the Wolffian duct and WT-1 and GDNF expressions in the metanephric blastema were initially normal in the mutant. The ureteric bud grew and invaded the metanephric mesenchyme where Pax-2 expression was normally induced. Subsequently, however, Pax-2, c-ret and Lim1 expressions in the ureteric bud and GDNF expression in the mesenchyme were greatly reduced. Wnt-4 expression was never found in the mesenchyme. The tip of the ureteric bud never dilated and branching of the bud did not occur. Neither pretubular cell aggregates nor epithelialization were found in the mesenchyme. Instead the ureteric bud soon degenerated and apoptotic figures were prominent in mesenchymal cells. In explant culture, the mutant ureteric bud did not induce the epithelial transformation of the wild-type mesenchyme, and branching of the mutant ureteric bud was not induced by wild-type mesenchyme. In contrast, defects were not apparent in the mutant mesenchyme by co-culture with wild-type ureteric bud or spinal cord. These results suggest that, in metanephrogenesis, Emx2 is essential for the ureteric bud functions subsequent to Pax-2 induction in the metanephric mesenchyme. Degeneration of the Wolffian duct and mesonephric tubules was also abnormally accelerated without the formation of the Mullerian duct.

Emx1 and Emx2 functions in development of dorsal telencephalon

The genes Emx1 and Emx2 are mouse cognates of a Drosophila head gap gene, empty spiracles, and their expression patterns have suggested their involvement in regional patterning of the forebrain. To define their functions we introduced mutations into these loci. The newborn Emx2 mutants displayed defects in archipallium structures that are believed to play essential roles in learning, memory and behavior: the dentate gyrus was missing, and the hippocampus and medial limbic cortex were greatly reduced in size. In contrast, defects were subtle in adult Emx1 mutant brain. In the early developing Emx2 mutant forebrain, the evagination of cerebral hemispheres was reduced and the roof between the hemispheres was expanded, suggesting the lateral shift of its boundary. Defects were not apparent, however, in the region where Emx1 expression overlaps that of Emx2, nor was any defect found in the early embryonic forebrain caused by mutation of the Emx1 gene, of which expression principally occurs within the Emx2-positive region. Emx2 most likely delineates the palliochoroidal boundary in the absence of Emx1 expression during early dorsal forebrain patterning. In the more lateral region of telencephalon, Emx2-deficiency may be compensated for by Emx1 and vice versa. Phenotypes of newborn brains also suggest that these genes function in neurogenesis corresponding to their later expressions.

Abscopal regression of hepatocellular carcinoma after radiotherapy for bone metastasis

Spontaneous regression of hepatocellular carcinoma is a rare phenomenon. Abscopal regression of tumours resulting from the effect of irradiation of a tissue on a remote non-irradiated tissue is also rare. The case of a 76 year old Japanese man with hepatocellular carcinoma that regressed after radiotherapy for thoracic vertebral bone metastasis is described. Serum levels of tumour necrosis factor-alpha increased after radiotherapy. The findings suggests that such abscopal related regression may be associated with host immune response, involving cytokines such as tumour necrosis factor-alpha.

Visceral endoderm mediates forebrain development by suppressing posteriorizing signals

The anterior visceral endoderm (AVE) has attracted recent attention as a critical player in mouse forebrain development and has been proposed to act as "head organizer" in mammals. However, the precise role of the AVE in induction and patterning of the anterior neuroectoderm is not yet known. Here we identified a 5'-flanking region of the mouse Otx2 gene (VEcis) that governs the transgene expression in the visceral endoderm. In transgenic embryos, VEcis-active cells were found in the distal visceral endoderm at 5.5 days postcoitus (dpc), had begun to move anteriorly at 5.75 dpc, and then became restricted to the AVE prior to gastrulation. The VEcis-active visceral endoderm cells exhibited ectodermal morphology distinct from that of the other endoderm cells and consisted of two cell layers at 5.75 dpc. In the Otx2(-/-) background, the VEcis-active endoderm cells remained distal even at 6.5 dpc when a primitive streak was formed; anterior definitive endoderm was not formed nor were any markers of anterior neuroectoderm ever induced. The Otx2 cDNA transgene under the control of the VEcis restored these Otx2(-/-) defects, demonstrating that Otx2 is essential to the anterior movement of distal visceral endoderm cells. In germ-layer explant assays between ectoderm and visceral endoderm, the AVE did not induce anterior neuroectoderm markers, but instead suppressed posterior markers in the ectoderm; Otx2(-/-) visceral endoderm lacked this activity. Thus Otx2 is also essential for the AVE to repress the posterior character. These results suggest that distal visceral endoderm cells move to the future anterior side to generate a prospective forebrain territory indirectly, by preventing posteriorizing signals.

Functional roles of Otx2 transcription factor in postnatal mouse retinal development

We previously reported that Otx2 is essential for photoreceptor cell fate determination; however, the functional role of Otx2 in postnatal retinal development is still unclear although it has been reported to be expressed in retinal bipolar cells and photoreceptors at postnatal stages. In this study, we first examined the roles of Otx2 in the terminal differentiation of photoreceptors by analyzing Otx2; Crx double-knockout mice. In Otx2+/-; Crx-/- retinas, photoreceptor degeneration and downregulation of photoreceptor-specific genes were much more prominent than in Crx-/- retinas, suggesting that Otx2 has a role in the terminal differentiation of the photoreceptors. Moreover, bipolar cells decreased in the Otx2+/-; Crx-/- retina, suggesting that Otx2 is also involved in retinal bipolar-cell development. To further investigate the role of Otx2 in bipolar-cell development, we generated a postnatal bipolar-cell-specific Otx2 conditional-knockout mouse line. Immunohistochemical analysis of this line showed that the expression of protein kinase C, a marker of mature bipolar cells, was significantly downregulated in the retina. Electroretinograms revealed that the electrophysiological function of retinal bipolar cells was impaired as a result of Otx2 ablation. These data suggest that Otx2 plays a functional role in the maturation of retinal photoreceptor and bipolar cells.

Canonical Wnt Signaling and Its Antagonist Regulate Anterior-Posterior Axis Polarization by Guiding Cell Migration in Mouse Visceral Endoderm

The mouse embryonic axis is initially formed with a proximal-distal orientation followed by subsequent conversion to a prospective anterior-posterior (A-P) polarity with directional migration of visceral endoderm cells. Importantly, Otx2, a homeobox gene, is essential to this developmental process. However, the genetic regulatory mechanism governing axis conversion is poorly understood. Here, defective axis conversion due to Otx2 deficiency can be rescued by expression of Dkk1, a Wnt antagonist, or following removal of one copy of the beta-catenin gene. Misexpression of a canonical Wnt ligand can also inhibit correct A-P axis rotation. Moreover, asymmetrical distribution of beta-catenin localization is impaired in the Otx2-deficient and Wnt-misexpressing visceral endoderm. Concurrently, canonical Wnt and Dkk1 function as repulsive and attractive guidance cues, respectively, in the migration of visceral endoderm cells. We propose that Wnt/beta-catenin signaling mediates A-P axis polarization by guiding cell migration toward the prospective anterior in the pregastrula mouse embryo.

External mechanical cues trigger the establishment of the anterior-posterior axis in early mouse embryos

Mouse anterior-posterior axis polarization is preceded by formation of the distal visceral endoderm (DVE) by unknown mechanisms. Here, we show by in vitro culturing of embryos immediately after implantation in microfabricated cavities that the external mechanical cues exerted on the embryo are crucial for DVE formation, as well as the elongated egg cylinder shape, without affecting embryo-intrinsic transcriptional programs except those involving DVE-specific genes. This implies that these developmental events immediately after implantation are not simply embryo-autonomous processes but require extrinsic factors from maternal tissues. Moreover, the mechanical forces induce a breach of the basement membrane barrier at the distal portion locally, and thereby the transmigrated epiblast cells emerge as the DVE cells. Thus, we propose that external mechanical forces exerted by the interaction between embryo and maternal uterine tissues directly control the location of DVE formation at the distal tip and consequently establish the mammalian primary body axis.

Competition for Mitogens Regulates Spermatogenic Stem Cell Homeostasis in an Open Niche

In many tissues, homeostasis is maintained by physical contact between stem cells and an anatomically defined niche. However, how stem cell homeostasis is achieved in environments where cells are motile and dispersed among their progeny remains unknown. Using murine spermatogenesis as a model, we find that spermatogenic stem cell density is tightly regulated by the supply of fibroblast growth factors (FGFs) from lymphatic endothelial cells. We propose that stem cell homeostasis is achieved through competition for a limited supply of FGFs. We show that the quantitative dependence of stem cell density on FGF dosage, the biased localization of stem cells toward FGF sources, and stem cell dynamics during regeneration following injury can all be predicted and explained within the framework of a minimal theoretical model based on "mitogen competition." We propose that this model provides a generic and robust mechanism to support stem cell homeostasis in open, or facultative, niche environments.

Developmental patterning and evolution of the mammalian viscerocranium: genetic insights into comparative morphology

The vertebrate cranium is generally classified into the neurocranium and the viscerocranium. The latter is derived from the neural crest and so is the prechordal portion of the neurocranium. A view we favor considers the prechordal neurocranium as the premandibular component of the viscerocranium, and the vertebrate skull to consist of the neural crest-derived viscerocranium and the mesodermal neurocranium. Of these developmental units, only the viscerocranium appears to have completely segmented metamerical organization. The Hox code which is known to function in specification of the viscerocranium does not extend rostrally into the mandibular and premandibular segments. By genetic manipulation of rostrally expressed non-Hox homeobox genes, the patterning mechanism of the head is now demonstrated to be more complicated than isomorphic registration of the Hox code to pharyngeal arches. The phenotype by haplo-insufficiency of Otx2 gene, in particular, implies the premandibular cranium shares a common specification mechanism with the mandibular arch. Our interpretation of the metamerical plan of the viscerocranium offers a new scheme of molecular codes associated with the vertebrate head evolution.

Dosage-dependent hedgehog signals integrated with Wnt/beta-catenin signaling regulate external genitalia formation as an appendicular program

Embryonic appendicular structures, such as the limb buds and the developing external genitalia, are suitable models with which to analyze the reciprocal interactions of growth factors in the regulation of outgrowth. Although several studies have evaluated the individual functions of different growth factors in appendicular growth, the coordinated function and integration of input from multiple signaling cascades is poorly understood. We demonstrate that a novel signaling cascade governs formation of the embryonic external genitalia [genital tubercle (GT)]. We show that the dosage of Shh signal is tightly associated with subsequent levels of Wnt/beta-catenin activity and the extent of external genitalia outgrowth. In Shh-null mouse embryos, both expression of Wnt ligands and Wnt/beta-catenin signaling activity are downregulated. beta-catenin gain-of-function mutation rescues defective GT outgrowth and Fgf8 expression in Shh-null embryos. These data indicate that Wnt/beta-catenin signaling in the distal urethral epithelium acts downstream of Shh signaling during GT outgrowth. The current data also suggest that Wnt/beta-catenin regulates Fgf8 expression via Lef/Tcf binding sites in a 3' conserved enhancer. Fgf8 induces phosphorylation of Erk1/2 and cell proliferation in the GT mesenchyme in vitro, yet Fgf4/8 compound-mutant phenotypes indicate dispensable functions of Fgf4/8 and the possibility of redundancy among multiple Fgfs in GT development. Our results provide new insights into the integration of growth factor signaling in the appendicular developmental programs that regulate external genitalia development.

Cell surface heparan sulfate chains regulate local reception of FGF signaling in the mouse embryo

Heparan sulfate (HS) proteoglycans modulate the activity of multiple growth factors on the cell surface and extracellular matrix. However, it remains unclear how the HS chains control the movement and reception of growth factors into targeted receiving cells during mammalian morphogenetic processes. Here, we found that HS-deficient Ext2 null mutant mouse embryos fail to respond to fibroblast growth factor (FGF) signaling. Marker expression analyses revealed that cell surface-tethered HS chains are crucial for local retention of FGF4 and FGF8 ligands in the extraembryonic ectoderm. Fine chimeric studies with single-cell resolution and expression studies with specific inhibitors for HS movement demonstrated that proteolytic cleavage of HS chains can spread FGF signaling to adjacent cells within a short distance. Together, the results show that spatiotemporal expression of cell surface-tethered HS chains regulate the local reception of FGF-signaling activity during mammalian embryogenesis.

Cooperation between Otx1 and Otx2 genes in developmental patterning of rostral brain

Otx1 and Otx2 genes are mouse cognates of a Drosophila head gap gene orthodenticle. The homozygous mutants have previously indicated that Otx2 is essential to development of structures anterior to rhombomere 3, probably reflecting its expression around the early primitive streak stage. Otx2 mutation also exhibits craniofacial defects by haplo-insufficiency. Affected structures correspond to the most anterior and most posterior parts of the Otx2 expression where Otx1 is not, or is only weakly, expressed at the time of brain regionalization. No apparent defects are found in early brain development by the Otx1 mutation, suggesting that the Otx1 and Otx2 functions overlap in the regions where both are expressed. To demonstrate this, the Otx1/Otx2 double heterozygous phenotype was examined in this study. Analyses with molecular markers at 9.5 days post coitus suggested the failure in development of mesencephalon and caudal diencephalon with the expansion of anterior metencephalon. Genes expressed in isthmus exhibited a characteristic lateral stripe normally, although rostrally shifted, except that Fgf8 expression was expanded dorsally. The defects were apparent at the 6-somite stage, but not at the 3-somite stage. Broad Fgf8 expression at the 3-somite stage took place normally, but it did not concentrate into a spot corresponding to future isthmus. The double heterozygous phenotype implicates a previously unsuspected mechanism for development of the mes/metencephalic territory; at the 3- to 6-somite stage Otx1 cooperates with Otx2 to establish the mes/diencephalic domain, allowing for the correct development of isthmus/ rhombomere 1.

Regulation of Otx2 expression and its functions in mouse forebrain and midbrain

Otx2 expression in the forebrain and midbrain was found to be regulated by two distinct enhancers (FM and FM2) located at 75 kb 5′upstream and 115 kb 3′ downstream. The activities of these two enhancers were absent in anterior neuroectoderm earlier than E8.0; however, at E9.5 their regions of activity spanned the entire mesencephalon and diencephalon with their caudal limits at the boundary with the metencephalon or isthmus. In telencephalon, activities were found only in the dorsomedial aspect. Potential binding sites of OTX and TCF were essential to FM activity, and TCF sites were also essential to FM2 activity. The FM2 enhancer appears to be unique to rodent; however, the FM enhancer region is deeply conserved in gnathostomes. Studies of mutants lacking FM or FM2 enhancer demonstrated that these enhancers indeed regulate Otx2 expression in forebrain and midbrain. Development of mesencephalic and diencephalic regions was differentially regulated in a dose-dependent manner by the cooperation between Otx1and Otx2 under FM and FM2 enhancers: the more caudal the structure the higher the OTX dose requirement. At E10.5 Otx1–/–Otx2ΔFM/ΔFMmutants, in which Otx2 expression under the FM2 enhancer remained,exhibited almost complete loss of the entire diencephalon and mesencephalon;the telencephalon did, however, develop.

Extracellular modulation of Fibroblast Growth Factor signaling through heparan sulfate proteoglycans in mammalian development

Fibroblast Growth Factor (FGF) signaling plays crucial roles in multiple cellular processes including cell proliferation, differentiation, survival, and migration during mammalian embryogenesis. In the extracellular matrix, as well as at the cell surface, the movement of FGF ligands to target cells and the subsequent complex formations with their receptors are positively and negatively controlled extracellularly by heparan sulfate proteoglycans (HSPGs) such as syndecans, glypicans, and perlecan. Additionally, spreading of HSPGs by cleavage with sheddases such as proteinases and heparanases, and the overall length and sulfation level of specific heparan sulfate structures further generate a great diversity of FGF signaling outcomes. This review presents our current understanding of the regulatory mechanisms of FGF signaling in extracellular spaces through HSPGs in mammalian development.

FGF signaling directs a center-to-pole expansion of tubulogenesis in mouse testis differentiation

In mouse embryogenesis, Sry is transiently activated in a center-to-pole wavelike manner along the anteroposterior (AP) axis of developing XY gonads. However, the mechanism and significance of the center-to-pole expansion of testis initiation pathways downstream of Sry expression remain unclear. Here we demonstrate that FGF9 can act as a diffusible conductor for a poleward expansion of tubulogenic programs at early phases of testis differentiation. In XY genital ridge cultures of anterior, middle and posterior segments at 11.0-11.25 days post-coitum, male-specific activation of Sry and its target gene, Sox9, was still observed in both anterior and posterior pole segments despite their isolation from the central domain. However, high-level Sox9 expression was not maintained, resulting in the failure of testis cord organization in most pole segments. A reconstruction experiment using ROSA:lacZ middle segments showed rescue of the tubulogenic defect in the poles without any appreciable contribution of lacZ-positive gonadal parenchyma cells. A partition culture assay also showed a possible contribution of soluble/diffusible factors secreted from the gonadal center domain to proper tubulogenesis in the poles. Among various signaling factors, Fgf9 expression was significantly lower in both anterior and posterior pole segments than in the central domain. The supportive role of the central domain could be substituted by exogenous FGF9 supply, whereas reduction of Wnt4 activity did not rescue the tubulogenesis defect in the pole segments. These observations imply that center-to-pole FGF9 diffusion directs a poleward expansion of testiculogenic programs along the AP axis of developing XY gonads.

Transcriptional regulatory networks in epiblast cells and during anterior neural plate development as modeled in epiblast stem cells

Somatic development initiates from the epiblast in post-implantation mammalian embryos. Recent establishment of epiblast stem cell (EpiSC) lines has opened up new avenues of investigation of the mechanisms that regulate the epiblast state and initiate lineage-specific somatic development. Here, we investigated the role of cell-intrinsic core transcriptional regulation in the epiblast and during derivation of the anterior neural plate (ANP) using a mouse EpiSC model. Cells that developed from EpiSCs in one day in the absence of extrinsic signals were found to represent the ANP of ~E7.5 embryos. We focused on transcription factors that are uniformly expressed in the E6.5 epiblast but in a localized fashion within or external to the ANP at E7.5, as these are likely to regulate the epiblast state and ANP development depending on their balance. Analyses of the effects of knockdown and overexpression of these factors in EpiSCs on the levels of downstream transcription factors identified the following regulatory functions: cross-regulation among Zic, Otx2, Sox2 and Pou factors stabilizes the epiblastic state; Zic, Otx2 and Pou factors in combination repress mesodermal development; Zic and Sox2 factors repress endodermal development; and Otx2 represses posterior neural plate development. All of these factors variably activate genes responsible for neural plate development. The direct interaction of these factors with enhancers of Otx2, Hesx1 and Sox2 genes was demonstrated. Thus, a combination of regulatory processes that suppresses non-ANP lineages and promotes neural plate development determines the ANP.

Overexpression of eNOS in NTS causes hypotension and bradycardia in vivo

The role of nitric oxide (NO) in the brain in the control of blood pressure and the sympathetic nervous system is debated. This study examined the effect of overexpression of endothelial NO synthase (eNOS) in the nucleus tractus solitarii (NTS) on blood pressure in conscious rats. Adenovirus vectors encoding either eNOS (AdeNOS) or ss-galactosidase were transfected into the NTS in vivo. In the AdeNOS-treated rats, the local expression of eNOS in the NTS was confirmed by immunohistochemical staining and Western blot analysis for the eNOS protein and by increased production of nitrite/nitrate in the NTS measured by in vivo microdialysis. Blood pressure and heart rate, monitored by the use of a radiotelemetry system in a conscious state, were significantly decreased in the AdeNOS-treated group at day 5 to day 10 after the gene transfer. Urinary norepinephrine excretion also was decreased at day 7 after the gene transfer in the AdeNOS-treated group. Our results indicate that overexpression of eNOS in the NTS decreases blood pressure, heart rate, and sympathetic nerve activity in conscious rats.

Dkk3-Cre BAC transgenic mouse line: a tool for highly efficient gene deletion in retinal progenitor cells

To establish the genetic tools for conditional gene deletion in mouse retinal progenitors, we generated a Dkk3-Cre transgenic mouse line using bacterial artificial chromosome (BAC) transgenesis. Cre recombination efficiency in vivo was assayed by crossing this transgenic line, termed BAC-Dkk3-Cre, with the CAG-CAT-Z reporter line. This BAC-Dkk3-Cre line showed Cre recombinase activity in most retinal progenitors. Cre activity was detectable from embryonic day 10.5 (E10.5) and generally restricted to the retina during embryogenesis. To verify that BAC-Dkk3-Cre mice successfully circumvented lethality, we generated Otx2flox/flox/BAC-Dkk3-Cre+ mice as Otx2 conditional knockout mice. The Otx2flox/flox/BAC-Dkk3-Cre+ mice were viable, and their retina showed loss of mature cell-type markers of photoreceptor cells, bipolar cells, and horizontal cells, in contrast, amacrine-like cells noticeably increased. Thus, the BAC-Dkk3-Cre transgenic mouse line provides a powerful tool for generating conditional knockout mouse lines for studying loss of gene functions in the developing retina.

Extracellular distribution of diffusible growth factors controlled by heparan sulfate proteoglycans during mammalian embryogenesis

During mouse embryogenesis, diffusible growth factors, i.e. fibroblast growth factors, Wnt, bone morphogenetic protein and Hedgehog family members, emanating from localized areas can travel through the extracellular space and reach their target cells to specify the cell fate and form tissue architectures in coordination. However, the mechanisms by which these growth factors travel great distances to their target cells and control the signalling activity as morphogens remain an enigma. Recent studies in mice and other model animals have revealed that heparan sulfate proteoglycans (HSPGs) located on the cell surface (e.g. syndecans and glypicans) and in the extracellular matrix (ECM; e.g. perlecan and agrin) play crucial roles in the extracellular distribution of growth factors. Principally, the function of HSPGs depends primarily on the fine features and localization of their heparan sulfate glycosaminoglycan chains. Cell-surface-tethered HSPGs retain growth factors as co-receptors and/or endocytosis mediators, and enzymatic release of HSPGs from the cell membrane allows HSPGs to transport or move multiple growth factors. By contrast, ECM-associated HSPGs function as a reservoir or barrier in a context-dependent manner. This review is focused on our current understanding of the extracellular distribution of multiple growth factors controlled by HSPGs in mammalian development.

Characterization of the pufferfish Otx2 cis-regulators reveals evolutionarily conserved genetic mechanisms for vertebrate head specification

The Otx2 gene, containing a highly conserved paired-type homeobox, plays a pivotal role in the development of the rostral head throughout vertebrates. Precise regulation of the temporal and spatial expression of Otx2 is likely to be crucial for proper head specification. However, regulatory mechanisms of Otx2 expression remain largely unknown. In this study, the Otx2 genome of the puffer fish Fugu rubripes, which has been proposed as a model vertebrate owing to its highly compact genome, was cloned. Consistently, Fugu Otx2 possesses introns threefold smaller in size than those of the mouse Otx2 gene. Otx2 mRNA was transcribed after MBT, and expressed in the rostral head region throughout the segmentation and pharyngula periods of wild-type Fugu embryos. To elucidate regulatory mechanisms of Otx2 expression, the expression of Otx2-lacZ reporter genes nearly covering the Fugu Otx2 locus, from -30.5 to +38.5 kb, was analyzed, by generating transgenic mice. Subsequently, seven independent cis-regulators were identified over an expanse of 60 kb; these regulators are involved in the mediation of spatiotemporally distinct subdomains of Otx2 expression. Additionally, these expression domains appear to coincide with local signaling centers and developing sense organs. Interestingly, most domains do not overlap with one another, which implies that cis-regulators for redundant expression may be abolished exclusively in the pufferfish so as to reduce its genome size. Moreover, these cis-regions were also able to direct expression in zebrafish embryos equivalent to that observed in transgenic mice. Further comparative sequence analysis of mouse and pufferfish intergenic regions revealed eight highly conserved elements within these cis-regulators. Therefore, we propose that, in vertebrate evolution, the Otx2 promoter acquires multiple, spatiotemporally specific cis-regulators in order to precisely control highly coordinated processes in head development.

Making very similar embryos with divergent genomes: conservation of regulatory mechanisms of Otx between the ascidians Halocynthia roretzi and Ciona intestinalis

Ascidian embryos develop with a fixed cell lineage into simple tadpoles. Their lineage is almost perfectly conserved, even between the evolutionarily distant species Halocynthia roretzi and Ciona intestinalis, which show no detectable sequence conservation in the non-coding regions of studied orthologous genes. To address how a common developmental program can be maintained without detectable cis-regulatory sequence conservation, we compared in both species the regulation of Otx, a gene with a shared complex expression pattern. We found that in Halocynthia, the regulatory logic is based on the use of very simple cell line-specific regulatory modules, the activities of which are conserved, in most cases, in the Ciona embryo. The activity of each of these enhancer modules relies on the conservation of a few repeated crucial binding sites for transcriptional activators, without obvious constraints on their precise number, order or orientation, or on the surrounding sequences. We propose that a combination of simplicity and degeneracy allows the conservation of the regulatory logic, despite drastic sequence divergence. The regulation of Otx in the anterior endoderm by Lhx and Fox factors may even be conserved with vertebrates.

Regulation ofOtx2expression and its functions in mouse epiblast and anterior neuroectoderm

We have identified cis-regulatory sequences acting on Otx2expression in epiblast (EP) and anterior neuroectoderm (AN) at about 90 kb 5′ upstream. The activity of the EP enhancer is found in the inner cell mass at E3.5 and the entire epiblast at E5.5. The AN enhancer activity is detected initially at E7.0 and ceases by E8.5; it is found later in the dorsomedial aspect of the telencephalon at E10.5. The EP enhancer includes multiple required domains over 2.3 kb, and the AN enhancer is an essential component of the EP enhancer. Mutants lacking the AN enhancer have demonstrated that these cis-sequences indeed regulate Otx2 expression in EP and AN. At the same time, our analysis indicates that another EP and AN enhancer must exist outside of the –170 kb to +120 kb range. In Otx2ΔAN/– mutants, in which one Otx2allele lacks the AN enhancer and the other allele is null, anteroposterior axis forms normally and anterior neuroectoderm is normally induced. Subsequently, however, forebrain and midbrain are lost, indicating that Otx2 expression under the AN enhancer functions to maintain anterior neuroectoderm once induced. Furthermore, Otx2 under the AN enhancer cooperates with Emx2 in diencephalon development. The AN enhancer region is conserved among mouse, human and Xenopus; moreover, the counterpart region in Xenopus exhibited an enhancer activity in mouse anterior neuroectoderm.